The present invention relates to diamino compounds which are useful for material monomers of high-molecular compounds such as polyimides, polyamides, epoxy resins and the like, crosslinking agents and modifiers, and to liquid crystal aligning films comprising polyimides which are obtained from the diamino compounds.
The main current of liquid crystal display elements which are used in conventional clocks, watches and electronic calculators is a twist nematic (abbreviated as TN hereinafter) mode having a structure in which molecular alignment of nematic liquid crystals is twisted at an angle of 90.degree. on the surface of a couple of upper and lower electrode substrates.
However, this display mode is insufficient to obtain improved display in quality and size because it shows an indistinct contrast and a narrow viewing angle.
In recent years, a liquid crystal display element using supertwist nematic (abbreviated as STN hereinafter) mode in which the molecular alignment of nematic liquid crystals is twisted at angles of 180-270 degrees between the upper and lower electrode substrates has been developed, and then large panel liquid crystal elements having sufficient display quality are developing. Among these elements, in an element having a relatively narrow twist angle (twisted at 180-200 degrees), surface treatment on the electrode substrates is sufficiently conducted in the similar manner to those employed in conventional TN cells which are equipped with aligning films having surface alignment of pretilt angles (abbreviated as .theta. hereinafter) of five degrees and below. In the specification, the pretilt angle means the angle between the rod-shaped liquid crystal molecules and the substrate of the liquid crystal cell. In STN modes having twist angles of 200-270 degrees along with better display quality, surface alignment having higher pretilt angles (10.degree.&lt;.theta..ltoreq.30.degree.) must be used, and therefore liquid crystal display cells having the aligning films which satisfy these angles are required.
In polyimide aligning films which are currently available for the TN mode, the limit of pretilt angles of display cells produced on a technical scale is about five degrees.
Japanese Publication of Unexamined Patent Application No. 61-240223 describes a liquid crystal display element which is equipped with liquid crystal aligning films produced from a polyimide resin.
The resin has a repeating unit represented by a formula: ##STR2## wherein R.sub.1 and R.sub.2 indicate the same or different groups selected from a hydrogen atom, an alkyl group having one to four carbon atoms and CF.sub.3, and R.sub.3 and R.sub.4 indicate the same or different groups selected from a hydrogen atom and an alkyl group having one to four carbon atoms. As materials for the polyimide resin, the only diamine represented by a formula: ##STR3## is exemplified.
However, the polyimide aligning films produced from the above diamine have a problem that high pretilt angles are unobtainable as shown in the comparative examples described hereinafter.
Furthermore, there are polyimide aligning films having high pretilt angles for the STN mode.
Problems of these films are stability and reproducibility of pretilt angles over the whole display surface of a cell substrate. In order to obtain the high pretilt angles definitely, the best method which is currently conducted is film formation by vacuum oblique evaporation of SiO and the like.
However, as the films are mass-produced by the vacuum evaporation, it is a costly process in its production unit.
As the result, it is desired earnestly to realize the aligning films having high pretilt angles by the surface treatment of rubbing organic films that is the same method as in the conventional surface treatment method which has been employed in the TN mode.